In such a braking system, normal braking is effected by means of the foot brake pedal, i.e. by operation of the first brake valve, the hand control being operated only in special situations or in an emergency. However, in practice, misuse of the hand control occurs. Such misuse occurs primarily in the case of freight companies which engage independent owner-operators to pull their trailers, the owner-operators conserving their own tractor brakes by making excessive use of the hand control rather than by applying normal braking using the foot brake pedal. This practice results in excessive wear on the trailer brakes, broken brake drums, and increased tire wear. Moreover, the practice is an impediment to the safe operation of the unit since the condition of trailer brakes which have been used improperly and excessively cannot be gauged by the standard procedure based on mileage.
The emergency/parking brakes of the trailer are usually fluid-operated spring brakes which, in normal road operating conditions are released from a locked condition by fluid supplied from the tractor, the brakes automatically being biased to the locked conditioning the event of loss of fluid pressure. For such a system the invention provides security means to prevent unauthorized release of the brakes when the trailer is parked. The invention also provides security means to prevent unauthorized release of the tractor parking brakes when the tractor is parked.